U.S. patent application number 12/416023 was filed with the patent office on 2010-09-30 for optical mode coupler.
This patent application is currently assigned to INFINERA CORPORATION. Invention is credited to Brent E. Little.
Application Number | 20100247037 12/416023 |
Document ID | / |
Family ID | 42784361 |
Filed Date | 2010-09-30 |
United States Patent
Application |
20100247037 |
Kind Code |
A1 |
Little; Brent E. |
September 30, 2010 |
OPTICAL MODE COUPLER
Abstract
An optical coupler includes a first waveguide configured to
supply a first optical signal having a wavelength and a second
waveguide. The first optical signal having a first mode. The first
waveguide has a tapered portion being spaced from the second
waveguide by a distance sufficient to facilitate evanescent
coupling of the first optical signal from the first waveguide to
the second waveguide. A first effective refractive index of the
first waveguide at a location in the tapered portion being equal to
a second effective refractive index at a location in the second
waveguide. The first effective refractive index being associated
with the first mode and the second effective refractive index being
associated with a second mode of a second optical signal having the
wavelength. The second mode having a different order than the first
mode, and the second waveguide being configured to supply the
second optical signal.
Inventors: |
Little; Brent E.; (Glen
Head, NY) |
Correspondence
Address: |
Infinera Corporation;Ross Carothers
1322 Bordeaux Drive
Sunnyvale
CA
94089
US
|
Assignee: |
INFINERA CORPORATION
Annapolis Juction
MD
|
Family ID: |
42784361 |
Appl. No.: |
12/416023 |
Filed: |
March 31, 2009 |
Current U.S.
Class: |
385/28 |
Current CPC
Class: |
G02B 2006/12145
20130101; G02B 6/1228 20130101; G02B 6/12007 20130101; G02B 6/14
20130101; G02B 2006/12152 20130101; G02B 6/125 20130101 |
Class at
Publication: |
385/28 |
International
Class: |
G02B 6/26 20060101
G02B006/26 |
Claims
1. An optical coupler, comprising: a first waveguide configured to
supply a first optical signal having a wavelength, the first
optical signal having a first mode; and a second waveguide, wherein
the first waveguide has a tapered portion, the tapered portion
being spaced from the second waveguide by a distance sufficient to
facilitate evanescent coupling of the first optical signal from the
first waveguide to the second waveguide, a first effective
refractive index of the first waveguide at a location in the
tapered portion being equal to a second effective refractive index
at a location in the second waveguide, the first effective
refractive index being associated with the first mode and the
second effective refractive index being associated with a second
mode of a second optical signal having the wavelength, the second
mode being different than the first mode, and the second waveguide
being configured to supply the second optical signal.
2. The coupler of claim 1, wherein the taper portion tapers from a
first width to a second width, the second width being smaller than
the first width.
3. The coupler of claim 1, wherein the second waveguide has a
second tapered portion tapering from a third width to a fourth
width.
4. The coupler of claim 3, wherein the third width is smaller than
the fourth width.
5. An optical coupler, comprising: a first coupling region,
including: a first portion of a first waveguide, the first portion
of the first waveguide adiabatically tapering from a first width to
a second width, the first waveguide configured to guide an optical
signal in a first mode; and a first portion of a second waveguide,
the first portion of the second waveguide having a third width, the
first portion of the second waveguide disposed adjacent to the
first portion of the first waveguide such that an optical signal
guided by the first portion of the first waveguide couples into the
first portion of the second waveguide in a second mode, wherein the
modes being coupled are of a different mode order.
6. The optical coupler of claim 5, wherein an effective index of a
mode in the first portion of the first waveguide increases within
the first coupling region from an effective index that is smaller
than an effective index of a mode in the first portion of the
second waveguide to an effective index that is greater than the
effective index of a mode in the first portion of the second
waveguide.
7. The optical coupler of claim 6, wherein the second guided mode
is of a lower order than the first guided mode.
8. The optical coupler of claim 5, wherein an effective index of a
mode in the first portion of the first waveguide decreases within
the first coupling region from an effective index that is larger
than an effective index of a mode in the first portion of the
second waveguide to an effective index that is less than the
effective index of a mode in the first portion of the second
waveguide.
9. The optical coupler of claim 8, wherein the second guided mode
is of a higher order than the first guided mode.
10. The optical coupler of claim 5, wherein the second waveguide
adiabatically tapers from the third width to a fourth width within
the first coupling region.
11. The optical coupler of claim 10, wherein the third width is
greater than the fourth width, and the second width is greater than
the first and third widths.
12. The optical coupler of claim 5, further comprising: a second
coupling region, including: a second portion of the first
waveguide, the second portion of the first waveguide having a
fourth width within the second coupling region; and a second
portion of the second waveguide, the second portion of the tapering
from a fifth width to a sixth width within the second coupling
region, the second portion of the second waveguide disposed
adjacent to the second portion of the first waveguide such that the
optical signal guided by the second portion of the second waveguide
couples into the second portion of the first waveguide in a third
guided mode.
13. The optical coupler of claim 12, wherein an effective index of
a mode in the second portion of the second waveguide decreases with
respect to an effective index of a mode in the second portion of
the first waveguide within the second coupling region.
14. The optical coupler of claim 12, wherein an effective index of
a mode in the second portion of the second waveguide increases with
respect to an effective index of a mode in the second portion of
the first waveguide within the second coupling region.
15. The optical coupler of claim 5, further comprising: a second
coupling region, including: a second portion of the second
waveguide, the second portion of the second waveguide tapering from
a fourth width to a fifth width within the second coupling region;
and a first portion of a third waveguide, the first portion of the
third waveguide disposed adjacent to the second portion of the
second waveguide such that the optical signal guided by the second
portion of the second waveguide couples into the first portion of
the third waveguide in a third guided mode.
16. The optical coupler of claim 15, wherein an effective index of
a mode in the second portion of the first waveguide decreases with
respect to an effective index of a mode in the second portion of
the second waveguide within the second coupling region.
17. The optical coupler of claim 15, wherein an effective index of
a mode in the second portion of the first waveguide increases with
respect to an effective index of a mode in the second portion of
the second waveguide within the second coupling region.
18. The optical coupler of claim 5, further comprising: a second
coupling region, including: a second portion of the second
waveguide having a fourth width; and a first portion of a third
waveguide tapering from a fifth width to a sixth width, the first
portion of the third waveguide disposed adjacent to the second
portion of the second waveguide such that the optical signal guided
by the second portion of the second waveguide couples into the
first portion of the third waveguide in a third guided mode.
19. The optical coupler of claim 18, wherein an effective index of
a mode in the first portion of the third waveguide increases with
respect to an effective index of a mode in the second portion of
the second waveguide within the second coupling region.
20. The optical coupler of claim 18, wherein an effective index of
a mode in the first portion of the third waveguide decreases with
respect to an effective index of a mode in the second portion of
the second waveguide within the second coupling region.
Description
FIELD OF DISCLOSURE
[0001] Embodiments of the invention relate to the field of optical
communication devices. More particularly, the present invention
relates to mode couplers for optical systems.
BACKGROUND
[0002] Energy associated with an optical signal may be spatially
distributed as the optical signal propagates through a waveguide.
Different distributions of such optical energy are often referred
to as "modes". In a fundamental mode, the optical energy has a
Gaussian distribution, whereby the optical energy is concentrated
in the center of the waveguide and tapers off toward the edges of
the waveguide. Higher order modes, such as second, third, etc.
modes are associated with different spatial distributions of the
optical energy.
[0003] Optical signals having multiple modes can simultaneously
propagate in a waveguide. Conventionally, such multimode signals
are generated by passing an optical signal having a signal mode or
fundamental mode through a grating. The grating is specifically
manufactured such that certain predefined modes are created in the
waveguide as the initial optical signal passes through the grating.
However, these gratings need to be precisely manufactured to tight
tolerances. Otherwise, an optical signal with the desired modes
will not be produced. For example, if the grating spacing is note
precisely maintained, when the optical signal passes through the
grating, undesired modes may be created. Additionally, such
fabrication induced errors may result in power loss due to
scattering.
[0004] Accordingly, an improved system of combining multiple modes
of an optical signal is desirable.
SUMMARY
[0005] In one embodiment, an optical coupler includes a first
waveguide configured to supply a first optical signal having a
wavelength and a second waveguide. The first optical signal having
a first mode. The first waveguide has a tapered portion that is
spaced from the second waveguide by a distance sufficient to
facilitate evanescent coupling of the first optical signal from the
first waveguide to the second waveguide. A first effective
refractive index of the first waveguide at a location in the
tapered portion being equal to a second effective refractive index
at a location in the second waveguide. The first effective
refractive index being associated with the first mode and the
second effective refractive index being associated with a second
mode of a second optical signal having the wavelength. The second
mode being different than the first mode, and the second waveguide
being configured to supply the second optical signal.
[0006] In another embodiment, an optical coupler includes a
coupling region. The coupling region includes a first portion of a
first waveguide and a first portion of a second waveguide. The
first portion of the first waveguide adiabatically tapers from a
first width to a second width within -the first coupling region.
The first waveguide is configured to guide an optical signal in a
first mode. The first portion of the second waveguide has a third
width and is disposed adjacent to the first portion of the first
waveguide such that an optical signal guided by the first portion
of the first waveguide couples into the first portion of the second
waveguide in a second mode. The modes being coupled are of a
different mode order.
BRIEF DESCRIPTION OF THE DRAWINGS
[0007] FIG. 1 illustrates one example of an optical mode
coupler.
[0008] FIGS. 2A and 2B are simulated couplings of a fundamental
mode being coupled into a first higher order mode in accordance
with the optical mode coupler illustrated in FIG. 1.
[0009] FIG. 2C is a graph of effective index of the waveguide modes
versus distance of the waveguide modes in the optical mode coupler
shown in FIG. 2A.
[0010] FIG. 3A is a simulated coupling of a fundamental mode of an
optical signal in a second waveguide being coupled into and
exciting a second higher order mode of a second waveguide in
accordance with the optical mode coupler illustrated in FIG. 1.
[0011] FIG. 3B is a graph of effective index of the waveguide modes
versus distance of the waveguide modes in the optical mode coupler
shown in FIG. 3A.
[0012] FIG. 4A is a simulated coupling of a first higher order mode
of an optical signal in a first waveguide being coupled into and
exciting a second higher order mode of a second waveguide in
accordance with the optical mode coupler illustrated in FIG. 1.
[0013] FIG. 4B is a graph of effective index of the waveguide modes
versus distance of the waveguide modes in the optical mode coupler
shown in FIG. 4A.
[0014] FIG. 5A illustrates one example of a cascade of optical mode
couplers.
[0015] FIG. 5B is a simulation of a fundamental mode of an optical
signal in a first waveguide being changed into a second higher
order mode in accordance with the cascade of optical mode couplers
illustrated in FIG. 5A.
DETAILED DESCRIPTION
[0016] Consistent with the present disclosure, an optical mode
coupler includes a portion of first and second waveguides within a
coupling region. An optical signal having a first mode traveling in
the first waveguide is coupled into a second waveguide within the
coupling region. The effective refractive indices of the first
and/or second waveguides are adjusted within the coupling region
such that when optical signal is coupled into the second waveguide
it has a second mode that is of a different order than the first
mode.
[0017] FIG. 1 illustrates one example of an optical mode coupler
100. As shown in FIG. 1, the optical mode coupler 100 includes a
first waveguide 102 and a second waveguide 104 that extend from an
isolation region 106 into a coupling region 108. The waveguides may
then separate again forming a second isolation region 110. In one
embodiment, the first waveguide 102 may be a tap waveguide
configured to tap all or a portion of an optical signal traveling
in the second waveguide 104 into the first waveguide 102. In other
embodiments, the first waveguide may be an input waveguide
receiving an optical input signal, which is then coupled into the
second waveguide 102. As shown in FIG. 1, the first waveguide. 102
may have an initial width, W.sub.1, at a first end 102a and a
second width, W.sub.2, at a second or output end 102b. Preferably,
the first waveguide 102 tapers in a direction extending from first
end 102a to second end 102b. The second waveguide 104, however, may
have a substantially constant width, W.sub.3, from a first end 104a
to a second end 104b. The width of the second waveguide 104 is
sized such that second waveguide may support multiple modes of
light at a given wavelength. In the first and second isolation
regions 106, 110, the first and second waveguides 102, 104 are
sufficiently spaced apart to prevent evanescent coupling of optical
signals between the waveguides. Accordingly, the effective indices
of the first and second waveguides 102, 104 may be adjusted within
the isolations regions 106, 110 without causing an optical signal
to couple from one waveguide into another waveguide.
[0018] Within the coupling region 108, the waveguides are spaced
from each other such that an optical signal may be evanescently
coupled from one waveguide, e.g., first waveguide 102, into the
other waveguide, e.g., second waveguide 104. In the isolation
regions 106, 110, the first and second waveguides 102, 104 are
spaced such that an optical signal typically does not evanescently
couple from one waveguide to another.
[0019] As generally understood, each of the modes or set of modes
in the first and second waveguides 102, 104 has an effective
refractive index that is based on the actual refractive index of
each waveguide as well as other physical parameters including the
width of the waveguide. To couple an optical signal from the first
waveguide 102 into the second waveguide 104 and excite a higher
order mode in the second waveguide 104, the effective refractive
index of the mode in the first waveguide 102 is reduced with
respect to the effective refractive index of the mode in the second
waveguide 104 within the coupling region 108. This may be
accomplished in a variety of ways. For example, the width of the
first waveguide 102 may be decreased while the width of the second
waveguide 104 is held constant as illustrated in FIG. 1, the width
of the second waveguide 104 may be increased and the width of the
first waveguide 102 may be held constant, or the width of the first
waveguide 102 may be decreased while the width of the second
waveguide 104 is simultaneously increased. The effective refractive
index of the waveguide may also be changed by decreasing the
waveguide height by etching, polishing, or selective growth of the
waveguide. Additionally, the effective refractive index of the
waveguide may be changed by modifying the material index through
dopant concentrations, bleaching, selective growth, thermal-optical
effects, electro-optic effects, or the like.
[0020] As illustrated in FIG. 1, the widths of the waveguides may
be adjusted by tapering the waveguides within the coupling region
108. Although the first waveguide 102 in FIG. 1 is illustrated as
linearly tapering, it will be understood that the taper may also be
non-linear. Implementing the taper such that it is adiabatic
prevents optical signal power from being lost due to radiation or
signal scattering.
[0021] FIG. 2A shows a simulation of an optical mode coupler in
accordance with the coupler illustrated in FIG. 1 in which a
fundamental mode, M.sub.0, in the first waveguide 102 is coupled
into the second waveguide 104 as an optical signal of a first
higher order mode, N.sub.1. FIG. 2B shows a simulation of an
optical mode coupler in accordance with the coupler illustrated in
FIG. 1 in which the first waveguide 102 is a tap waveguide. The
first waveguide 102 tapped a portion of the power of the optical
signal in the second waveguide 104, e.g., 10%, and couples it back
into the second waveguide 104, possibly after a dispersive element
(e.g., a ring resonator or a delay line not shown in FIG. 2B)
induced a frequency-dependent phase shift in the optical signal in
the first waveguide 102. As can be seen in FIG. 2B, the coupler 100
may be used to create optical signals having any number of combined
modes that may be phase-shifted with respect to each other.
[0022] FIGS. 2A and 2B were simulated for an optical signal having
a wavelength of 1.55 .mu.m propagating in a waveguide core having a
refractive index of 1.65 and the surrounding cladding having a
refractive index of 1.45. The width, W.sub.3, of the second
waveguide 104 was 2 .mu.m, the initial width, W.sub.1, of the first
waveguide 102 was 1.5 .mu.m, and the final width, W.sub.2, of the
first waveguide 102 was 0.6 .mu.m. The length, L, of the adiabatic
taper was 500 .mu.m.
[0023] Advantageously, the length of the adiabatic taper, L, may be
varied without having a detrimental effect on the performance of
the coupler 100. For example, FIG. 2C is a graph of effective
refractive index of the waveguide modes versus distance within the
coupling region 108 of the coupler illustrated in FIG. 1. As shown
in FIG. 2C, the adiabatic taper length, L, is greater than the
coupling distance, D.sub.c, e.g., the distance at which the optical
signal is fully coupled into the second waveguide 104. The line
N.sub.0 in FIG. 2C corresponds to the effective refractive index of
the fundamental mode in the second waveguide 104. Similarly, line
N.sub.1 corresponds to the effective refractive indices of the
first higher order mode of the second waveguide 104, and line
N.sub.2 corresponds to the effective refractive index of the second
higher order mode of the second waveguide 104. The line M.sub.0(W)
corresponds to the effective refractive index of the fundamental
mode of the first waveguide 102 as it varies as a function of
width, W.
[0024] The dotted line in FIG. 2C illustrates the effective
refractive index of the optical signal as it transitions from the
first waveguide 102 into the second waveguide 104. As the width of
the first waveguide 102 is reduced, the effective refractive index
of the mode in the first waveguide 102 is also reduced. As the
effective refractive index of the mode in the first waveguide 102
continues to reduce it eventually becomes less than the effective
refractive index of the first higher order mode of the second
waveguide, N.sub.1. At this point, e.g., at coupling distance
D.sub.c, the optical signal in the first waveguide 102 couples into
the second waveguide 104 as a first higher order mode optical
signal as seen in FIG. 2A since the first and second waveguides
102, 104 are close enough to enable evanescent coupling of optical
signals.
[0025] The optical signal couples into the second waveguide 104 as
an optical signal of a higher order mode, N.sub.1, and not the
fundamental mode N.sub.0, because the effective index of the
optical signal in the first waveguide 102 is smaller than the
effective index of the fundamental mode, N.sub.0, in the second
waveguide 104. The optical signal will only couple as a next
available mode, e.g., mode N.sub.1, and not a second higher order
mode, N.sub.2, as light does not jump or skip an available mode.
Accordingly, the manufacturing tolerances for a coupler 100 may be
reduced compared to conventional mode couplers as the length of the
adiabatic taper may be varied without creating an undesired mode or
modes as is a common problem for conventional couplers.
[0026] An optical signal may be able to be coupled from a
fundamental mode in the first waveguide 102 into a second higher
order mode in the second waveguide 104 by varying the effective
refractive indices of the modes of waveguides 102, 104. For
example, the effective refractive index of the first mode in the
first waveguide 102, M.sub.0(W), may be reduced in the isolation
region 106 from a point between the fundamental mode, N.sub.0, and
the first higher order mode, N.sub.1, of the second waveguide 104
to a point between the first higher order mode, N.sub.1, and the
second higher order mode, N.sub.2, of the second waveguide 104.
Then, the effective index, M.sub.0(W), of the first waveguide 102
may be further reduced in the coupling region 108 such that it
crosses the effective index of the effective index of the second
higher order mode, N.sub.2, at which point the optical signal will
be coupled from a fundament mode in the first waveguide 102 into
the second higher order mode of the second waveguide 104.
[0027] Coupler 100 may also be configured to create an optical
signal having a mode of a higher order than the first higher order
mode, N.sub.1, of the second waveguide 104. For example, coupler
100 may be designed such that a second (N.sub.2), third (N.sub.3),
fourth (N.sub.4), or other higher order mode of the second
waveguide 104 is excited. FIG. 3A is a simulation showing a
fundamental mode, M.sub.0, of the first waveguide 102 being coupled
into the second waveguide 104 and exciting the second higher order
mode, N.sub.2, of the second waveguide 104. The simulation was
performed using a first waveguide 102 and an optical signal having
the same or similar characteristics as the first waveguide 102 and
optical signal described above with reference to FIGS. 2A and 2B.
The width, W.sub.c, of the second waveguide was increased to 4.2
.mu.m. FIG. 3B is a graph of effective refractive indices versus
distance illustrating the evolution of the optical signal
illustrated in FIG. 3A.
[0028] Optical signals of a higher order than the fundamental mode
(e.g., N.sub.1, N.sub.2, N.sub.3, etc.) in the first waveguide 102
may be used to generate higher order modes (e.g., M.sub.1, M.sub.2,
M.sub.3, etc.) in the second waveguide 104 as well as lower order
modes such as the fundamental mode, M.sub.0. FIG. 4A is a
simulation showing a first higher order mode, M.sub.1, of the first
waveguide 102 generating a second higher order mode, N.sub.2, of
the second waveguide 104. The simulation was performed using a
first waveguide 102 having an initial width, W.sub.1, of 3.0 .mu.m,
a final width, W.sub.2, of 1.7 .mu.m, and a length, L, of the
adiabatic taper of 700 .mu.m. The second waveguide 104 has a width,
W.sub.c, of 4.2 .mu.m and the optical signal had the same
characteristics of the optical signal described above with respect
to FIGS. 2A and 2B. FIG. 4B is a graph of effective refractive
indices of modes versus distance illustrating the evolution of the
optical signal illustrated in FIG. 4B.
[0029] Multiple couplers 100 may be cascaded to convert an optical
signal of a first mode into an optical signal of a second mode in
the same waveguide. FIG. 5A illustrates one example of a mode
coupler 200. As shown in FIG. 5A, the first waveguide 202 has a
first width, W.sub.1, that adiabatically tapers to a second width
W.sub.2. The second width, W.sub.2, adiabatically tapers to a third
width, W.sub.3, which then adiabatically tapers to a fourth width,
W.sub.4. The second waveguide 204 adiabatically tapers from a fifth
width, W.sub.5, to a sixth width, W.sub.6. The sixth width then
tapers to a seventh width, W.sub.7.
[0030] The mode coupler 200 illustrated in FIG. 5A is configured to
couple an optical signal of a first mode in the second waveguide
204 into the first waveguide 202 in a first coupling region 206
such that the resultant optical signal is of a higher order. The
higher order optical signal traveling along the first waveguide 202
is then coupled back into the second waveguide 204 in a second
coupling region 210 at yet another higher order. One skilled in the
art will understand that other mode couplers 200 may be designed by
varying the adiabatic tapers of the first and second waveguides
202, 204. As shown in FIG. 5A, the widths of the first and second
waveguides 202, 204 may be adjusted within the isolation region 208
to achieve the desired effective refractive index for coupling the
optical signal into the desired mode in the next coupling region
210.
[0031] FIG. 5B is a simulation of the mode converter 200
illustrated in FIG. 5A. The simulation was performed using an
optical signal having a wavelength of 1.55 .mu.m in a waveguide
core having a refractive index of 1.65 and the surrounding cladding
having a refractive index of 1.45. The second waveguide 204 had an
initial width, W.sub.5, of 1.5 .mu.m that adiabatically tapered to
a second width, W.sub.6, of 0.6 .mu.m over a length of 700 .mu.m.
The sixth width, W.sub.6, tapered to a seventh width, W.sub.7, of
0.6 .mu.m. The first waveguide 102 had an initial width, W.sub.1,
of 2.3 .mu.m that adiabatically tapered to a second width, W.sub.2,
of 2.7 .mu.m. The second width, W.sub.2, of the first waveguide 202
adiabatically tapered to a third width, W.sub.3, of 2.0 .mu.m. The
second waveguide 202 then tapered from the third width, W.sub.3, of
2.0 .mu.m to a fourth width, W.sub.4, of 0.6 .mu.m.
[0032] The couplers described above advantageously enable specific
modes to be generated in a waveguide. The couplers may have relaxed
tolerances compared to conventional couplers implemented using
gratings as the couplers described herein are less prone to
fabrication induced errors such as power loss and inadvertent mode
excitation.
[0033] Although the disclosure has been described in terms of
exemplary embodiments, it is not limited thereto. Rather, the
appended claims should be construed broadly, to include other
variants and embodiments of the disclosure, which may be made by
those skilled in the art without departing from the scope and range
of equivalents of the disclosure.
* * * * *